1. Field of the Invention
This invention relates generally to a digital signal transmitting system for use with, for example, a wire or cable television system having many television receivers as terminals and, in particular, is directed to a digital signal transmitting system in which descramble control data is transmitted to each television receiver by using a service bit signal.
2. Description of the Prior Art
A digital signal transmitting system has been proposed for transmitting a digital signal reproduced from a so-called compact disc or the like by using a CATV (cable television) transmission line. Such a system is disclosed, for example, in U.S. Pat. No. 4,656,629 ("Digital Signal Transmitting and/or Receiving System") filed Nov. 6, 1984, and which is assigned to the same assignee as the present application.
The above-mentioned digital signal transmitting system includes a signal generator for producing time division-multiplexed digital audio signals and service bit signals, a modulator for modulating a carrier in accordance with the time division-multiplexed signals and a transmitter to transmit the modulated carrier through a CATV transmission line. At a receiver or terminal, the service bit signals are decoded so as to control the channel change-over or selection of a television receiver to a vacant channel when an emergency broadcast signal is transmitted. A muting control circuit is also provided to mute an audio signal from a D/A (digital-to-analog) converter.
However, in such previously proposed digital signal transmitting system, although an address signal of a terminal is time division-multiplexed on service bit signals and then transmitted, if the number of the terminals is increased, the addressing or accessing speed is lowered. Therefore, a system which can address a large number of terminals, for example, one million terminals, at high speed has not yet been proposed.
Further, in a pay channel CATV system, scrambled data is transmitted to the receiver, but it has not yet been proposed to transmit a descramble key code to each terminal together with an address number of that terminal.